Light detection and ranging (lidar) is a technique for remote imaging, in which the distance of an object is measured using laser light. A typical lidar system includes a rangefinder and a scanning system. The rangefinder, typically, includes a laser with a wavelength in the ultraviolet (UV) to near-infrared (NIR) range, as well as a receiver. The distance to an object can be determined by measuring the time taken for outgoing pulses of laser light to be reflected back into the receiver, or by measuring the phase shift between an outgoing and an incoming modulated beam of laser light. The scanning system, typically, includes a mirror system for scanning the field of view of the lidar system over the area to be imaged by reflecting the outgoing and incoming laser light.
To satisfactorily image static and moving objects by lidar in applications such as autonomous vehicle operation, collision avoidance, and surveillance, accurate scanning at a high repetition rate is necessary. For instance, even for a relatively modest image resolution of 144 lines at a frame rate of 5 Hz, 720 lines per second must be acquired. Therefore, mirror systems that are capable of accurately executing a scan pattern at high angular speeds are desired.
Conventionally, two main types of mirror system are applied in scanning systems for lidar, nodding-mirror systems and polygonal-mirror systems. Nodding-mirror systems include a nodding mirror, which is, typically, a planar mirror that rotates clockwise and counterclockwise through an angular range. Polygonal-mirror systems include a polygonal mirror, which is a faceted mirror shaped as a regular polygon. The polygonal mirror can be rotated clockwise or counterclockwise through 360°.
Nodding-mirror systems offer the advantage that, provided the nodding mirror is large enough, the efficiency of the nodding mirror in collecting the reflected light into the rangefinder, hereafter referred to as collection efficiency, is near-perfect over the entire angular range. In contrast, the collection efficiency is not uniform over all angular positions of the polygonal mirror. As the outgoing light from the rangefinder approaches the edges of the facets of the polygonal mirror, the collection efficiency drops to 50%. Furthermore, owing to manufacturing errors, the dimensions of the polygonal mirror may deviate from those of a regular polygon, leading to image distortion.
However, polygonal-mirror systems offer the advantage that once the polygonal mirror is set in rotation, the angular speed of the polygonal mirror must simply be maintained. In contrast, the rotation of the nodding mirror must be periodically stopped and reversed when the nodding mirror reaches the end of the angular range. Therefore, the angular speed at which the nodding mirror can be rotated through a scan pattern is, typically, lower than that of the polygonal mirror.
In a conventional scan pattern, the nodding mirror is rotated at a constant angular speed in one direction through an angular range and then rotated as fast as possible in the opposite direction to the start of the angular range. During the segment of the scan pattern in which the nodding mirror is returning to the start of the angular range, useful data is not being collected into the rangefinder. To optimally execute such a scan pattern, the nodding mirror should ideally be capable of rotating at high angular speeds and of undergoing rapid angular acceleration.
Nodding-mirror systems with a variety of configurations have been implemented in scanning systems for lidar, but these conventional nodding-mirror system have some important limitations.
Different types of rotary drives have been used to rotate the nodding mirror in such nodding-mirror systems. Nodding-mirror systems including stepper motors are disclosed in U.S. Pat. No. 5,337,189 to Krawczyk, et al. and U.S. Pat. No. 6,650,402 to Sullivan, et al., for example. However, these nodding-mirror systems have the disadvantage that the gear mechanism of the stepper motors introduces backlash, limiting the accuracy and the angular speed of the rotation of the nodding mirror. Nodding-mirror systems including galvanometer motors are disclosed in U.S. Pat. No. 5,006,721 to Cameron, et al., U.S. Pat. No. 7,135,672 to Land, and U.S. Pat. No. 7,215,430 to Kacyra, et al., for example. However, these nodding-mirror systems have the disadvantage that the galvanometer motors provide relatively low torque, limiting the size of the nodding mirror that can be rotated.
Such nodding-mirror systems may also include different types of detectors for ascertaining the angular position of the nodding mirror. Nodding-mirror systems including angular-position sensors are disclosed in U.S. Pat. No. 4,810,088 to Karning, et al. and U.S. Pat. No. 6,262,800 to Minor, for example. Nodding-mirror systems including rotary encoders are disclosed in U.S. Pat. No. 5,231,401 to Kaman, et al., U.S. Pat. No. 6,107,770 to Jackson, et al., and U.S. Pat. No. 7,215,430 to Kacyra, et al., for example. The output signals of such detectors, typically, serve as feedback to control circuitry for controlling the rotary drive. The effectiveness of the closed-loop feedback control is limited by the accuracy and resolution of the detectors.
An object of the present invention is to overcome the shortcomings of the prior art by providing a scanning system optimized for a lidar system. As part of such a scanning system for lidar, an optimized nodding-mirror system is provided that includes a nodding mirror, a rotary electromagnetic drive, a rotary optical encoder, and control circuitry. The rotary electromagnetic drive is lightweight, yet powerful enough to rotate a nodding mirror of the size preferred for lidar systems, at high angular speeds. The absence of mechanical linkages between the stationary yoke of the rotary electromagnetic drive and the moving arm, which is coupled to the nodding mirror, eliminates the possibility of any backlash and allows the direction of torque applied to the nodding mirror to be essentially instantly reversed. The rotary optical encoder of the nodding-mirror system has a high resolution and accuracy. Moreover, the detector of the rotary optical encoder and the encoder disk, which is coupled to the nodding mirror, are free of mechanical linkages, precluding backlash. The use of such a rotary optical encoder allows very accurate closed-loop feedback control of the scan pattern of the nodding mirror through control circuitry.
The unprecedented and advantageous combination of elements in the nodding-mirror system allows a variety of scan patterns to be accurately executed by the nodding mirror. Furthermore, additional mirror systems may be included in certain embodiments of the scanning system to increase the field of view of the lidar system.
An optimized polygonal-mirror system that includes a polygonal mirror, a rotary drive, a rotary encoder, and control circuitry is also provided as part of a scanning system for lidar. Advantageously, the control circuitry of the polygonal-mirror system is configured to modify the output signal of the rotary encoder to increase the resolution of the lidar system or to compensate for manufacturing defects in the polygonal mirror.